Ben,
I admire the fact that you want to get involved in artifacts. I have some pre-Colombian Mayan stuff but only one piece completely intact. I really don't know enough about Roman statues to even make an educated guess on its origin or weather or not its authentic and or valuable. It would be safe to say that something like this deserves an experts opinion. In the same breathe I wouldn't purchase any artifact of value without proper paperwork. Do so only invites disaster. If you get it for a few bucks, maybe you can get it authenticated, your in the UK so everything is relatively close by you should have no problem find such a person. Now what would it cost you to get something like this evaluated, I also have no idea. Good luck coin bro.